The steering of a vehicle is performed by transmitting operation of a steering member performed by the driver (in general, rotational operation of a steering wheel) to a steering mechanism disposed at an exterior portion of a passenger compartment so as to turn steered road wheels (in general, left and right front road wheels) by the operation of the steering mechanism. As a steering system for enabling steering like this, there is a vehicle steering system in which a transfer ratio varying mechanism which can vary a rotation transfer ratio is provided in an intermediate position of a steering shaft which connects a steering member and a steering mechanism together, and the transmission of rotation from the steering member to the steering mechanism is made to be implemented by varying the rotation transfer ratio in a stepless fashion (for example, refer to JP-A-2005-344757).
The transfer ratio varying mechanism described in JP-A-2005-344757 is a differential gear mechanism which has an input sun gear which is provided circumferentially on an input shape lying on the steering member side, an output sun gear disposed on the same axis as the input sun gear, an input planetary gear and an output planetary gear which mesh with the input sun gear and the output sun gear, respectively, and a carrier which supports these planetary gears via a common shaft, and the transfer ratio varying mechanism includes a transfer ratio control motor for causing the carrier to rotate around the axis.
According to this configuration, when the carrier is caused to rotate by driving the transfer ratio control motor, rotation of the input shaft that is inputted into the input sun gear is transmitted to the output sun gear and the output shaft via the input planetary gear and the output planetary gear which rotate on their own axes while rotating together with the carrier, and the rotation transfer ratio between the input and output shafts can be varied in a stepless fashion in accordance with an increase or decrease in the rotational speed of the carrier which is driven by the transfer ratio control motor. The transfer ratio control motor can also be used as a means for applying steering effort to the steering mechanism and is used to correct steering to eliminate an unstable steering condition attributed to lateral wind acting on a running vehicle or slippage of the road wheel/s on the road surface by allowing the transfer ratio control motor to operate irrespective of the operation of the steering member.